Method and system for automatic anti-tachycardia pacing

ABSTRACT

A method and system for delivering anti-tachycardia pacing is disclosed. A cardiac rhythm management device, such as an implantable pacemaker having anti-tachycardia pacing capability, delivers anti-tachycardia pacing therapy in accordance with an anti-tachycardia pacing protocol upon detection of a terminable arrhythmia. The anti-tachycardia pacing is delivered as a burst of one or more pacing pulses at a specified coupling interval after a sensed ventricular polarization. By sensing if an evoked potential occurs, the device can determine whether or not the anti-tachycardia pacing burst has captured the ventricle and can adjust the coupling interval and/or other parameters accordingly.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/037,302, filed on Oct. 25, 2001, the specification of which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention pertains to methods and system for treating cardiacarrhythmias with anti-tachycardia pacing.

BACKGROUND

Tachyarrhythmias are abnormal heart rhythms characterized by a rapidheart rate, typically expressed in units of beats per minute (bpm).Examples of tachyarrhythmias include supraventricular tachycardias(SVT's) such as sinus tachycardia, atrial tachycardia, and atrialfibrillation. The most dangerous tachyarrythmias, however, areventricular tachycardia (VT) and ventricular fibrillation (VF).Ventricular rhythms occur when re-entry of a depolarizing wavefront inareas of the ventricular myocardium with different conductioncharacteristics becomes self-sustaining or when an excitatory focus inthe ventricle usurps control of the heart rate from the sinoatrial node.The result is rapid and irregular contraction of the ventricles out ofelectromechanical synchrony with the atria. Most ventricular rhythmsexhibit an abnormal QRS complex in an electrocardiogram because they donot use the normal ventricular conduction system, the depolarizationspreading instead from the excitatory focus or point of re-entrydirectly into the myocardium. Ventricular tachycardia is typicallycharacterized by distorted QRS complexes that occur at a rapid rate,while ventricular fibrillation is diagnosed when the ventricledepolarizes in a chaotic fashion with QRS complexes of constantlychanging shape. Both ventricular tachycardia and ventricularfibrillation are hemodynamically compromising, and both can belife-threatening. Ventricular fibrillation, however, causes circulatoryarrest within seconds and is the most common cause of sudden cardiacdeath.

Cardioversion (an electrical shock delivered to the heart synchronouslywith the QRS complex) and defibrillation (an electrical shock deliveredwithout synchronization to the QRS complex to terminate ventricularfibrillation) can be used to terminate most tachyarrhythmias, includingSVT's, VT, and VF. The electric shock terminates the tachyarrhythmia bydepolarizing all of the myocardium simultaneously and rendering itrefractory. A class of cardiac rhythm management devices known as animplantable cardioverter/defibrillator (ICD) provides this kind oftherapy by delivering a shock pulse to the heart when the device detectsfibrillation.

Another type of electrical therapy for tachycardia is antitachycardiapacing (ATP). In ATP, the heart is competitively paced with one or morepacing pulses in an effort to interrupt the reentrant circuit causingthe tachycardia. Modern ICD's typically have ATP capability so that ATPtherapy is delivered to the heart when a tachycardia is detected, whilea shock pulse is delivered when fibrillation occurs. Althoughcardioversion/defibrillation will terminate tachycardia, it consumes alarge amount of stored power from the battery and results in patientdiscomfort owing to the high voltage of the shock pulses. It isdesirable, therefore, for the ICD to use ATP to terminate atachyarrhythmia whenever possible. It is commonly believed that onlycardioversion/defibrillation will terminate fibrillation and certainhigh rate tachycardias, while ATP can be used to treat lower ratetachycardias. A tachyarrhythmia that is regarded as terminable by ATPtherapy, based upon rate or other factors, will be referred to herein aseither a terminable tachyarrhythmia or a tachycardia.

In most ICD's with ATP capability, ventricular fibrillation (VF) isdistinguished from ventricular tachycardia (VT) using rate-basedcriteria so that ATP or shock therapy can be delivered as appropriate.The heart rate is usually measured by detection of the time betweensuccessive R waves (i.e., ventricular depolarizations). A measured heartrate is classified as a tachycardia when the rate is in a VT zone,defined as a range of rates above a tachycardia detection rate (TDR) butbelow a fibrillation detection rate (FDR). A measured heart rate abovethe FDR, on the other hand, is in the VF zone and is classified asfibrillation. In a typical device, a tachyarrhythmia with a heart ratein the VT zone is treated with ATP therapy in order to avoid anunnecessary painful shock to the patient, and a defibrillation shock isdelivered if the pacing fails to terminate the tachyarrhythmia. It is aprimary objective of the present invention to provide a method andapparatus for delivering ATP therapy in a manner that increases thelikelihood that ATP therapy will terminate a tachyarrhythmia withoutresorting to a defibrillation shock. The approach described below usescapture verification to determine whether a pacing pulse delivered inaccordance with a particular ATP protocol captures the myocardium sothat appropriate adjustments to the protocol can be made.

SUMMARY OF THE INVENTION

ATP therapy is only effective when the ATP pacing pulses actuallycapture the myocardium. Present ATP protocols, however, are open loopsystems that do not use any type of feedback to the device to confirmthat capture has occurred. The present invention is a method and devicefor the delivery of anti-tachycardia pacing (ATP) therapy upon detectionof a tachyarrhythmia with verification of capture by the pacing pulses.To deliver ventricular ATP therapy, a burst of one or more pacing pulsesis delivered in accordance with a particular ATP protocol, where theburst is output after a specified coupling interval with respect to aventricular sense. By sensing whether an evoked response occurs during acapture detection window following the output of a pacing pulse, it isdetermined whether the pulse has captured the ventricle. Such captureverification can then be used to adaptively adjust the value of thecoupling interval, cycle length, or other ATP parameters, thus improvingthe outcome of ATP protocols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a cardiac rhythm management device with ATPand cardioversion/defibrillation capability.

FIGS. 2A and 2B are flow diagrams showing the steps performed in aparticular implementation.

DETAILED DESCRIPTION

In the description that follows, a microprocessor-based cardiac rhythmmanagement device will be referred to as incorporating the system andmethod that is the present invention. In the embodiment to be described,the invention is implemented with a control unit made up of amicroprocessor executing programmed instructions in memory. It should beappreciated, however, that certain functions of a cardiac rhythmmanagement device could be controlled by custom logic circuitry eitherin addition to or instead of a programmed microprocessor. The term“controller” as used herein should therefore be taken to encompasseither custom circuitry (i.e., dedicated hardware) or a microprocessorexecuting programmed instructions contained in a processor-readablestorage medium along with associated circuit elements.

1. Hardware Platform

FIG. 1 is a system diagram of a microprocessor-based cardiac rhythmmanagement device with the capability of deliveringcardioversion/defibrillation shocks as well as antitachycardia pacingtherapy. The device may also be configured to deliver conventional(e.g., bradycardia) pacing as well. The controller 10 of the pacemakeris a microprocessor that communicates with a memory 12 via abidirectional data bus. The memory 12 typically comprises a ROM(read-only memory) for program storage and a RAM (random-access memory)for data storage. The pacemaker has atrial and ventricularsensing/pacing channels that respectively include electrodes 24 and 34,leads 23 and 33, sensing amplifiers 21 and 31, pulse generators 22 and32, and ventricular channel interfaces 20 and 30. A dedicated evokedresponse sensing channel is also provided that includes electrode 44,lead 43, sensing amplifier 41, and atrial channel interface 40.Incorporated into each sensing/pacing channel is thus a pacing channelmade up of the pulse generator connected to the electrode and a sensingchannel made up of the sense amplifier connected to the electrode. Aswitching network controlled by the microprocessor may be used to switchthe electrodes from the input of a sense amplifier to the output of apulse generator. In this embodiment, a single electrode is used forsensing and pacing in each channel, known as a unipolar lead. Otherembodiments may employ bipolar leads that include two electrodes foroutputting a pacing pulse and/or sensing intrinsic activity. The channelinterfaces communicate bidirectionally with microprocessor 10 andinclude analog-to-digital converters for digitizing sensing signalinputs from the sensing amplifiers and registers that can be written toby the microprocessor in order to adjust the gain and threshold valuesfor the sensing amplifiers. In the case of the atrial and ventricularchannels, registers can be written to in order to output pacing pulsesand change the pacing pulse amplitude and/or duration. A telemetryinterface 40 is also provided for communicating with an externalprogrammer 500 that has an associated display 510.

The controller 10 controls the overall operation of the device inaccordance with programmed instructions stored in memory, includingcontrolling the delivery of paces via the pacing channels, interpretingsense signals received from the sensing channels, and implementingtimers for defining escape intervals and sensory refractory periods. Thesensing circuitry of the pacemaker detects a chamber sense when a sensesignal (i.e., a voltage sensed by an electrode representing cardiacelectrical activity, sometimes called an electrogram signal) generatedby a particular channel exceeds a specified intrinsic detectionthreshold. A chamber sense may be either an atrial sense or aventricular sense depending on whether it occurs in the atrial orventricular sensing channel. Pacing algorithms used in particular pacingmodes employ such senses to trigger or inhibit pacing. Both bradycardiaand anti-tachycardia pacing modes may be implemented in code executed bythe controller.

2. Antitachycardia Pacing

The cardiac rhythm management device of FIG. 1 may be programmed with aplurality of selectable ATP pacing protocols that define the manner inwhich anti-tachycardia pacing is delivered. In a microprocessor-baseddevice, the output of pacing pulses is controlled by a pacing routinethat implements the selected pacing protocol as defined by variousparameters. A data structure stored in memory contains the parametersets that define each of the available pacing protocols. Pacingprotocols for ATP therapy can generally be divided into two classes:those that deliver one or more pulses in timed relation to detecteddepolarizations and those that deliver a continuous pulse train for aspecified time beginning after a detected depolarization. Both types ofATP protocols attempt to block the reentrant depolarization wavefrontcausing the tachycardia with a second depolarizing wavefront produced bya pacing pulse. Protocols of the first group may vary according toparameters that define the number of pulses delivered and the particulartiming employed. Protocols of the second group include so-called burstpacing in which a short train of pulses is delivered for a specifiedtime and may vary according to parameters that define the duration,frequency, and timing of the pulses.

The device delivers ATP therapy or a defibrillation shock underprogrammed control of the microprocessor in response to sensed activityfrom the sensing channels. A sensing routine analyzes the electricalactivity received from the sensing channels in order to detect atachyarrhythmia, and the tachyarrhythmia is then classified as atachycardia (i.e., a terminable tachyarrhythmia) or fibrillation basedupon rate and/or other criteria. The device detects a ventriculartachyarrhythmia, for example, by counting ventricular senses receivedvia the ventricular sensing channel in order to measure the heart rateand determine whether the rate exceeds a selected threshold value. Oncea tachyarrhythmia is detected, the rhythm is classified into either atachycardia or a fibrillation zone by comparing the heart rate to afibrillation rate boundary or by other means such as assessing thestability of the rhythm. If the tachyarrhythmia is classified asterminable, a pacing routine executed by the microprocessor delivers ATPpulses in accordance with the parameters of a selected protocol.

As noted above, the object of anti-tachycardia pacing is to create apace-induced wavefront that propagates into the re-entrant circuit ofthe tachycardia and extinguishes it. Different protocols are apt to bemore successful than others in terminating particular tachyarrhythmiasthat may differ as to rate and/or depolarization pattern. For thisreason, modern cardiac rhythm management devices are capable ofemploying a number of different ATP protocols to deliver therapy. Pacingparameters affecting the magnitude and timing of the pulses can also beadjusted for each protocol. In order for a pacing pulse to have anyeffect, the pulse must capture the ventricle so that a propagatingdepolarization results. This is complicated by the fact that during aventricular tachyarrhythmia, the action potential consumes a largeportion of the total cycle length, leaving only a small window of timewhen the ventricle is non-refractory and even less time for an induceddepolarization wavefront to propagate into the re-entrant circuit.

Ideally, a clinician would program the device to deliver pacing therapyusing a protocol and parameters that will perform best for a particularpatient's tachyarrhythmia. However, this may be difficult to predict, sothat a conventional technique for dealing with this problem is toprogram the device to deliver a number of ATP pacing bursts usingdifferent protocols and/or adjustable pacing parameters. One suchadjustable parameter is the coupling interval, which is the time fromthe last sensed depolarization to the first pacing pulse of a burst,commonly selected to be between 120 and 750 milliseconds. For capture tobe achieved by that pacing pulse, the end of the coupling interval mustoccur when the ventricle is non-refractory. In a so-called scan mode,some devices vary the coupling interval of a series of bursts in apredetermined manner. When the ATP pacing burst consists of a train ofpulses, the time between the pulses or cycle length is another parameterthat can be adjusted as in a ramp-type burst where the cycle lengthincreases or decreases with each pulse of the train.

3. Capture Verification

Conventional devices, however, do not obtain any information about how aparticular ATP pulse or group of pulses affected the heart other thanwhether or not a tachyarrhythmia was terminated. It would be useful forthe device to know if a particular ATP pulse was successful in capturingthe heart since that information could be used to further adjust certainATP parameters. According to the present invention, sensed electricalactivity in a heart chamber resulting from a pace, referred to as anevoked response, is used to verify that capture was achieved. An evokedresponse sensing channel, which may be a dedicated channel or asensing/pacing channel normally used to output pacing pulses and/orsense intrinsic activity, is used determine whether the pacing pulse hascaptured the heart chamber by detecting whether an evoked responseoccurs as a result of a pacing pulse. The particular channel used forevoked response detection should be one whose electrode is disposed in alocation where an evoked response due to the pacing electrode can bemost easily sensed. A ventricular sensing/pacing channel or a dedicatedevoked response sensing channel with an electrode disposed in the pacedventricle, for example, could be used to detect evoked responses toventricular paces.

In order to detect an evoked response, the sense signal generated by theevoked response sensing channel after a pacing pulse is compared with anevoked response detection threshold, which may be the same or differentas the intrinsic detection threshold used to detect chamber senses. Theevoked response detection threshold may also be adaptively adjusted asdescribed in U.S. Pat. No. 6,192,275 issued to Zhu et al., and assignedto Cardiac Pacemakers, Inc., which is hereby incorporated by reference.The comparison between the sense signal and the evoked responsedetection threshold takes place within a defined period of timefollowing output of the pacing pulse, referred to herein as a capturedetection window. After a pacing pulse is output, an evoked response iseither detected or not, signifying the presence or loss of capture,respectively.

Sensing channels are normally rendered refractory (i.e., insensitive)for a specified time period immediately following a pace in order toprevent the pacemaker from mistaking a pacing pulse or afterpotentialfor an intrinsic beat. To implement this function, the pacemakercontroller ignores what would otherwise be detected chamber senses inthe channel during the refractory interval. If the same sensing channelis used for both sensing intrinsic activity and evoked responses in achamber, the capture detection window is then further defined as aperiod that supercedes the normal refractory period so that thepacemaker is sensitive to an evoked response even if no intrinsic eventscan be detected. For example, a ventricular sensing/pacing channel maybe used to deliver ventricular paces, sense intrinsic ventricular beats,and detect evoked responses. During the capture detection windowfollowing a ventricular pace, the controller is prevented from detectinga ventricular sense but can still detect an evoked response if the sensesignal exceeds the evoked response detection threshold.

It is also common practice to block the sensing amplifier of a sensingchannel from receiving sense signals for a defined period of time thatstarts with a pacing pulse that is delivered through the same or adifferent channel, referred to as blanking. This is done in order toprevent saturation of the amplifier by the high voltage signal resultingfrom a pacing pulse. A separate period of time that overlaps the earlypart of a refractory interval is therefore defined, called a blankinginterval, during which the sense amplifiers are effectively disabled. Ifa blanking interval is employed in an evoked response sensing channel,the blanking interval is followed by a capture detection window duringwhich an evoked response may be detected by the evoked response sensingchannel. In an exemplary embodiment, the blanking period may beapproximately 10 ms, and the width of the capture detection window mayrange from 50 to 350 ms.

4. ATP with Capture Verification

Capture verification can be useful in delivering ATP therapy in a numberof different ways. For example, capture verification can be used asfeedback with the controller programmed to automatically adapt thecoupling interval to assure that the first pulse of a burst captures theventricle, thus eliminating the need to program a series of scannedbursts. The adaptation algorithm may operate so that the shortestpossible coupling interval that still captures is found. Captureverification can also be used to assure that capture is maintainedduring a burst consisting of a pulse train. For example, in a rampburst, the cycle length is progressively shortened from one pulse to thenext, and capture verification can determine when the cycle lengthbecomes too short and capture is lost. Once it is determined that thepulses are no longer capturing, the device can either terminate theburst or increase the cycle length. Capture verification can also beemployed on a beat-to-beat basis to assure that capture is maintained atthe fastest rate possible.

Capture verification can be used as part of a feedback system forautomatically determining the best ATP protocol and/or parameter valuesto be used in a particular patient. For example, after selecting an ATPprotocol and delivering a pacing burst, the controller counts andrecords the number of pacing pulses that failed to capture theventricle. Future protocol selection can then be at least partiallybased upon which anti-tachycardia pacing protocol had the fewest numberof recorded capture failures. The controller may also be programmed toautomatically optimize the parameters of a given protocol based uponcapture verification. For example, the controller may be programmed touse a particular ATP protocol with the cycle length and/or couplinginterval initialized to preset values and to vary these parameters inaccordance with the results of capture verification tests.

FIGS. 2A and 2B are flow diagrams showing the steps performed by acardiac rhythm management device in one particular implementation of theinvention. Referring first to FIG. 2A, the device is set up fordelivering anti-tachycardia pacing therapy at step S2 where a particularATP protocol is selected and various pacing parameter values are set,including the coupling interval CI. In this embodiment, the couplinginterval is initially set to a specified minimum value CI_(min).Clinician input for the set up procedure may be received via telemetryis received at step S1. At step S3, the device begins monitoringelectrical activity in a ventricle via a sensing channel and countsventricular senses to determine the ventricular rate. Using a rate-basedcriterion, the ventricular rate is classified as a terminabletachyarrhythmia when it falls within a specified zone. If a terminabletachyarrhythmia is detected at step S4, the device begins to deliver ATPtherapy. The device then waits for the next ventricular sense at step S5and starts a timer for the coupling interval CI. After expiration of thecoupling interval, an ATP burst is delivered at step S6. As the term isused herein, a burst may consist of only one pacing pulse or a series ofpacing pulses separated by a time interval referred to as the cyclelength. In the latter case, the coupling interval is measured withrespect to the initial pulse of the series. At step S7, the device looksfor an evoked response after the initial (or sole) pulse of the burstthrough a ventricular sensing channel in order to determine if the pulsecaptured the ventricle. If capture is achieved as determined at step S8,the device returns to step S3. If the tachyarrhythmia has persisted, theprocess is then repeated. If the ATP burst did not achieve capture, thecoupling interval CI is increased at step S9 before returning to stepS3. By increasing the coupling interval, the burst is moved away fromthe refractory period caused by the preceding intrinsic depolarization.Once capture is achieved, the coupling interval is maintained constantfor any subsequent bursts needed to terminate the tachyarrhythmia. Inthis implementation, the coupling interval is thus automatically set atthe minimum interval needed to achieve capture.

FIG. 2B shows in more detail the steps performed at step S6 when thedevice delivers a series-type burst having multiple pacing pulses andcapture is verified for each of pulse. At step S6 a, a pulse counter PCis set to a value N representing the number of pulses in the series asdefined by the protocol. At step S6 b, the initial pacing pulse of theseries is output after the coupling interval CI with respect to theprevious intrinsic depolarization, and any subsequent pulses are outputat a specified cycle length with respect to the previous pulse. Thepulse counter PC is also decremented by one. After each pulse is output,an evoked response is looked for at step S6 c. If capture is determinedto have occurred at step S6 d, the device then optionally decreases thecycle length if the selected ATP protocol is a ramp-type burst. Ifcapture did not occur, the device optionally either increases the cyclelength or sets the pulse counter PC to zero in order to terminate theburst. Next, the device tests the pulse counter at step S6 f to see ifthe series of pulses has been completed. If not, the device returns tostep S6 b, and the steps are repeated. Otherwise, the device proceeds tostep S7.

The above description has dealt with detecting ventricular tachycardiasand delivering ATP therapy to the ventricles. Although not commonlyemployed at the present time, ATP therapy can be used to terminateatrial tachyarrhythmias. It should be appreciated that the invention maybe used in conjunction with the delivery of ATP therapy to any heartchamber.

Although the invention has been described in conjunction with theforegoing specific embodiments, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

1-24. (Canceled)
 25. A method for delivery of anti-tachycardia pacing(ATP) therapy by a cardiac rhythm management device, comprising:generating sense signals representing electrical activity in a heartchamber and detecting a chamber sense when a sense signal exceeds aspecified intrinsic detection threshold; delivering a burst of one ormore pacing pulses in accordance with an anti-tachycardia pacingprotocol; determining if one or more pacing pulses of the burst havecaptured the heart chamber; and, adjusting a timing parameter of theburst in accordance with the capture determination.
 26. The method ofclaim 25 wherein the heart chamber is a ventricle.
 27. The method ofclaim 25 wherein the burst is output after a specified coupling intervalwith respect to a chamber sense further comprising increasing thecoupling interval when an initial pacing pulse of a burst has notcaptured the heart chamber.
 28. The method of claim 25 where the burstis output after a specified coupling interval with respect to a chambersense and further comprising: delivering a burst with the couplinginterval set to a specified minimum value; and, increasing the couplinginterval when an initial pacing pulse of a burst has not captured theheart chamber.
 29. The method of claim 25 wherein the delivered burst isa train of pacing pulses separated by a specified cycle length andfurther comprising terminating the burst when a pacing pulse fails tocapture the heart chamber.
 30. The method of claim 29 wherein the burstis a ramp-type burst such that the cycle length between pacing pulses isprogressively shortened with each pulse in the burst.
 31. The method ofclaim 25 wherein the delivered burst is a train of pacing pulsesseparated by a specified cycle length and further comprising increasingthe cycle length when a pacing pulse fails to capture the heart chamber.32. The method of claim 25 further comprising selecting ananti-tachycardia pacing protocol and recording the number of pacingpulses that failed to capture the heart chamber when a burst inaccordance with the protocol is delivered.
 33. The method of claim 25further comprising selecting an anti-tachycardia pacing protocol withthe fewest number of recorded capture failures.
 34. The method of claim25 further comprising determining whether a pacing pulse has achievedcapture by detecting whether an evoked response occurs during a capturedetection window following the output of a pacing pulse.
 35. A cardiacrhythm management device, comprising: a sensing channel for generatingsense signals representing electrical activity in a heart chamber; apacing channel for delivering paces to a selected heart chamber; acontroller for controlling the delivery of pacing pulses in accordancewith a programmed mode, detecting a chamber sense when the sense signalexceeds a specified intrinsic detection threshold, and detecting anevoked response when the sense signal following a pace exceeds aspecified evoked response detection threshold; and, wherein thecontroller is further programmed to deliver a burst of one or morepacing pulses to the heart chamber in accordance with ananti-tachycardia pacing protocol upon detection of a tachycardia,determine whether a pacing pulse of the burst has captured the heartchamber, and adjust a timing parameter of the burts in accordance withthe capture determination.
 36. The device of claim 35 wherein thesensing and pacing channel are adapted to sense and pace a ventricle.37. The device of claim 35 wherein the controller is programmed todeliver the burst after a specified coupling interval with respect to achamber sense and to increase the coupling interval when an initialpacing pulse of a burst has not captured the heart chamber.
 38. Thedevice of claim 35 wherein the controller is programmed to deliver theburst after a specified coupling interval with respect to a chambersense and further programmed to: deliver a burst with the couplinginterval set to a specified minimum value; and, increase the couplinginterval when an initial pacing pulse of a burst has not captured theheart chamber.
 39. The device of claim 35 wherein the controller isprogrammed to deliver the burst as a train of pacing pulses separated bya specified cycle length and to terminate the burst when a pacing pulsefails to capture the heart chamber.
 40. The device of claim 39 whereinthe controller is programmed to deliver the burst as a ramp-type burstsuch that the cycle length between pacing pulses is progressivelyshortened with each pulse in the burst.
 41. The device of claim 35wherein the controller is programmed to deliver the burst as a train ofpacing pulses separated by a specified cycle length and to increase thecycle length when a pacing pulse fails to capture the heart chamber. 42.The device of claim 25 wherein the controller is programmed to select ananti-tachycardia pacing protocol and record the number of pacing pulsesthat failed to capture the heart chamber when a burst in accordance withthe protocol is delivered.
 43. The device of claim 35 wherein thecontroller is programmed to select an anti-tachycardia pacing protocolwith the fewest number of recorded capture failures.
 44. The device ofclaim 35 wherein the controller is programmed to determine whether apacing pulse has achieved capture by detecting whether an evokedresponse occurs during a capture detection window following the outputof a pacing pulse.